Oilfield drilling fluid, often called “mud,” serves multiple purposes in the industry. Among its many functions, the drilling mud acts as a lubricant to cool rotary drill bits and facilitate faster cutting rates. The mud is mixed at the surface and pumped downhole at high pressure to the drill bit through a bore of the drillstring. Once the mud reaches the drill bit, it exits through various nozzles and ports where it lubricates and cools the drill bit. After exiting through the nozzles, the “spent” fluid returns to the surface through an annulus formed between the drillstring and the drilled wellbore.
Drilling mud provides a column of hydrostatic pressure, or head, to prevent “blow out” of the well being drilled. This hydrostatic pressure offsets formation pressures thereby preventing fluids from blowing out if pressurized deposits in the formation are breeched. Two factors contributing to the hydrostatic pressure of the drilling mud column are the height (or depth) of the column (i.e., the vertical distance from the surface to the bottom of the wellbore) itself and the density (or its inverse, specific gravity) of the fluid used. Depending on the type and construction of the formation to be drilled, various weighting and lubrication agents are mixed into the drilling mud to obtain the right mixture. Drilling mud weight is reported in “pounds,” short for pounds per gallon. Generally, increasing the amount of weighting agent solute dissolved in the mud base will create a heavier drilling mud. Drilling mud that is too light may not protect the formation from blow outs, and drilling mud that is too heavy may over invade the formation. Therefore, much time and consideration is spent to ensure the mud mixture is optimal. Because the mud evaluation and mixture process is time consuming and expensive, drillers and service companies prefer to reclaim the returned drilling mud and recycle it for continued use.
Another purpose of the drilling mud is to carry the cuttings away from the drill bit at the bottom of the borehole to the surface. As a drill bit pulverizes or scrapes the rock formation at the bottom of the borehole, small pieces of solid material are left behind. The drilling fluid exiting the nozzles at the bit acts to stir-up and carry the solid particles of rock and formation to the surface within the annulus between the drillstring and the borehole. Therefore, the fluid exiting the borehole from the annulus is a slurry of formation cuttings in drilling mud. Before the mud can be recycled and re-pumped down through nozzles of the drill bit, the cutting particulates have to be removed.
One type of apparatus used to remove cuttings and other solid particulates from drilling mud is commonly referred to in the industry as a “shale shaker” A shale shaker, also known as a vibratory separator, is a vibrating sieve-like table upon which returning used drilling mud is deposited and through which substantially cleaner drilling mud emerges. The shale shaker is an angled table with a generally perforated filter screen bottom. Returning drilling mud is deposited at the top of the shale shaker. As the drilling mud travels down the incline toward the lower end, the fluid falls through the perforations to a reservoir below thereby leaving the solid particulate material behind. The combination of the angle of inclination with the vibrating action of the shale shaker table enables the solid particles left behind to flow until they fall off the lower end of the shaker table.
The above described apparatus is illustrative of one type of shale shaker known to those of ordinary skill in the art. In alternate shale shakers, the top edge of the shaker may be relatively closer to the ground than the lower end. In such shale shakers, the angle of inclination may require the movement of particulates in a generally upward direction. In still other shale shakers, the table may not be angled, thus the vibrating action of the shaker alone may enable particle/fluid separation. Regardless, table inclination and/or design variations of existing shale shakers should not be considered a limitation of the present disclosure.
The amount of vibration and the angle of inclination of the shale shaker table are adjustable to accommodate various drilling mud flow rates and particulate percentages in the drilling mud. After the fluid passes through the perforated bottom of the shale shaker, it may either return to service in the borehole immediately, be stored for measurement and evaluation, or pass through an additional piece of equipment (e.g., a centrifuge or a smaller sized shale shaker) to remove smaller cuttings and/or particulate matter.
Because shale shakers are in continuous use, repair operations, and associated downtimes, need to be minimized as much as possible. Often, the filter screens of shale shakers, through which the solids are separated from the drilling mud, wear out over time and subsequently require replacement. Therefore, shale shaker filter screens are constructed to be quickly removable and easily replaceable. Generally, through the loosening of several bolts, the filter screen may be lifted out of the shaker assembly and replaced within a matter of minutes. While there are numerous styles and sizes of filter screens, they generally follow similar design.
Filter screens include a perforated plate base upon which a wire mesh, and/or other perforated filter overlay, is positioned. The perforated plate base generally provides structural support and allows the passage of fluids therethrough. While many perforated plate bases are flat or slightly arched, it should be understood that perforated plate bases having a plurality of corrugated or pyramid-shaped channels extending thereacross may be used instead. Pyramid-shaped channels may provide additional surface area for the fluid-solid separation process while guiding solids along their length toward the end of the shale shaker from where they are disposed.
In some shale shakers, a fine screen cloth is used with the vibrating screen. The screen may have two or more overlying layers of screen cloth and/or mesh. Layers of cloth or mesh may be bonded together and placed over a support, supports, or a perforated or apertured plate. The frame of the vibrating screen is resiliently suspended or mounted upon a support and is caused to vibrate by a vibrating mechanism (e.g., an unbalanced weight on a rotating shaft connected to the frame). Each screen may be vibrated by vibratory equipment to create a flow of trapped solids on top surfaces of the screen for removal and disposal of solids. The fineness or coarseness of the mesh of a screen may vary depending upon mud flow rate and the size of the solids to be removed.
FIG. 1 shows a conventional vibratory separator apparatus that includes a lower frame 12 and an upper basket 14. The vibratory separator apparatus 10 may have a variety of shapes and configurations, but generally it is intended to receive solids-laden mud from a distribution box (not shown) into the basket 14 that is vibrated by a motor (not shown) relative to the frame 12. The basket 14 includes an upstream end 18, a downstream end 20, a back wall 22 at the upstream end 18, and two side walls 24. The downstream end 20 is open. In operation, drilling mud including suspended solids is poured into the basket 14 over the back wall 22 and onto screen 16. Once on the screen 16, the solids-laden mud is vibrated toward the downstream end 20, which causes the mud to pass through the screen 16 into a collection box (not shown), and out of the vibratory separator apparatus 10 for further processing. The flow of the solids-laden mud is indicated at 25 in FIG. 1. The solids continue to be conveyed downstream on the screen 16 toward the open end 26 where they are either dropped onto another screen for further separation or discarded.
Screen 16 may be mounted in the basket 14 with wedges 30a that are hammered into place under wedge angles 32 that are welded to the inside of basket 14 at an angle corresponding to the angle of the wedge 30a. In this manner, the screens 16 were installed by placing a pre-tensioned screen 16 onto support rails (not shown) in basket 14. Once in place, a wedge 30a is placed on top of the pre-tensioned screen 16 under wedge angle 32 and then hammered into engagement with the wedge angle 32 to apply a downward force on the screen 16. Accordingly, contact between the screen 16 and the support rail (not shown) in basket 14 may be maintained.
A vibratory separator has multiple screens which are held in place by two wedges, one on each side of each screen. Changing the screens on a vibratory separator having multiple screens is a time consuming process for removing multiple wedges for each screen. Accordingly, there exists a need to reduce the amount of time required for screen changes.